Herpes Simplex virus (HSV) causes several serious human diseases including a sexually transmitted disease that significantly effects maternal and child health and may be a cofactor in the acquisition of other STDs such as AIDS. Once infected with HSV, individuals retain the virus in a latent state, from which periodic reactivation causes recurrent disease and provides an opportunity for transmission. The means to protect against HSV infection by inducing potent immune responses that can act quickly at the site of infection remains elusive. Replication-defective mutants of HSV offer a safer alternative to live virus vaccines. Because the infection does not spread in the host, these viruses also provide a unique tool to examine the induction of antiviral immune responses. Mice immunized subcutaneously or intranasally with an HSV-2 mutant virus generate immune responses that protect mice from disease and lethal infection upon genital challenge with a heterologous HSV-2 strain. It is hypothesized that immunization with replication-defective HSV-2 generates an immune response protective against genital HSV-2 that is principally mediated by CD4+ Th1 cells. In addition, it is also hypothesized that the provision of B7 co-stimulatory molecules in infected cells will augment the immunogenicity of the replication-defective virus and improve protective capacity. In Aim 1, histochemical analysis of the vaginal mucosa and the analysis of latent genomes in the ganglia will identify the stage of pathogenesis that is blocked by the protective immune response. In Aim 2, the components of the immune response that are protective against genital challenge will be identified using knockout mice, T cell subset depletions, and cytokine analyses. In Aim 3, the role of costimulation in the induction of immune response to replication-defective virus will be investigated by construction of recombinant viruses that encode B7 molecules. This information will facilitate developent of immunization strategies against HSV and possibly other pathogens of the genital tract.